Effect of visible light to the retina has been studied by electron microscopy. The following results have been obtained: 1) Phagocytic activity of the pigment epithelium against latex spheres is influenced by the size of the sphere. 2) Phagocytic activity of the pigment epithelium following light damage of the retina has been studied by exposing rats to fluorescent lamps at 150 foot-candles. Phagocytic activity is first accelerated by light, but eventually is diminished when the light damage becomes severe. Also, the phagocytic activity is controlled by the circadian rhythm. 3) Synaptic organs of the photoreceptor cells show marked cytologic changes by light exposure. One of the early changes is proliferation of the paramitochondrial membranes. The cone cells are resistant to light exposure. 4) Photoreceptor cells of the developing rat continue outer segment production under bright light (400 foot-candles) for more than two weeks without showing severe damage. Further exposure causes severe damage in the newly-formed outer segments. BIBLIOGRAPHIC REFERENCES: Funahashi, M., Okisaka, S. and Kuwabara, T.: Phagocytosis by the monkey pigment epithelium. Exp. Eye Res. 23: 217-225, 1976. Kuwabara, T. and Funahashi, M.: Light damage in the developing rat retina. Arch. Ophthalmol. 94: 1369-1374, 1976.